The physical DPI of my laptop screen is approximately 142 dots per inch as you can calculate from the correct output of xrandr --query:

$ xrandr --query | head
Screen 0: minimum 320 x 200, current 1920 x 1080, maximum 32767 x 32767
eDP1 connected primary 1920x1080+0+0 (normal left inverted right x axis y axis) 344mm x 193mm

However, Ubuntu appearantly forces the X server to use 96 dpi and thus assume a wrong physical size of the display, which you can check with xdpyinfo

$ xdpyinfo | grep -B2 dots
screen #0:
  dimensions:    1920x1080 pixels (508x286 millimeters)
  resolution:    96x96 dots per inch

As a consequence, fonts and other visual elements are smaller as they should be. You can check with any web browser on this website. The square is too small, less than 1 in^2.

This answer lists a number of work-arounds, none of which works for me:

  1. Tell lightdm to start X with the option -dpi 142, which I achieved on Saucy by editing /etc/lightdm/lightdm.conf.d/50-xserver-command.conf. Has no effect.

  2. Run xrandr -dpi 142x142, when starting the X session by adding a file to /etc/X11/Xsession.d/. Has no effect

  3. Set the text-scaling-factor, which can be achieved in a number of ways. This only effects fonts rendered by Unity/Gnome/GTK(?), basically the standard Ubuntu GUI, but fails for other apps such as web browsers or PDF viewers. Not what I want.

How do I get Ubuntu to use the correct dpi for my screen?

Update: This appears to be a known bug disguised as a feature. What are your workarounds?

2 Answers 2


To get current DPI resolution use

xdpyinfo | grep -B2 resolution

To update DPI resolution say to 142 dpi use

xrandr --dpi 142

I have install packet tracer and the variable QT_DEVICE_PIXEL_RATIO has defined :


i have comment in /etc/profile file :


it's ok now

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